Once in the preferences menu, click the audio/midi tab option, set audio output and audio input to Scarlett Solo USB.

Please exit preferences.

If your Scarlett Solo is set to Direct Monitor 'ON' on the front of the interface, you will want to switch Garagebands Input Monitoring off.If Direct Monitor is set to 'OFF', Garagebands Input Monitoring will need to be on in order to monitor your signal.

Make sure that you have selected the input on your Scarlett that you wish to use when creating a new track (input 1 on the Scarlett Solo = microphone only, input 2 = line/instrument). Turn up the input gain on your Solo to a desirable level (make sure halos are not in the red).